Due to turbulence and other flow irregularities the wind turbine rotor experiences large yawing moments. In trying to orient the rotor to the wind the yaw system of a wind turbine has to overcome these large yawing moments in addition to any friction in the yaw bearing. When trying to retain the rotor in its correctly oriented position the yaw system is exposed to large oscillating loads.
The mechanical systems for yawing a wind turbine generally consist of a gearing ring or the like which is usually fixed to the tower and of a drive system which is usually fixed to the bottom side, i.e. the bedplate, of a nacelle of the wind turbine. The drive system typically consists of a number of gear motors each having an output shaft with a pinion that engages with the gearing ring. The motors are typically electrical motors but may also be hydraulic motors.
The yaw drive system may be rated to drive the yawing movement of the nacelle under all conditions. However, on large wind turbines this means that the yaw drive system has to be of very substantial dimensions since turbulence eddies or other flow irregularities may occasionally give rise to very large yawing moments on the turbine rotor and hence the nacelle. Therefore, in modern wind turbines the yaw drive system is usually rated to provide a yawing moment that is adequate under all usual operating conditions, and it is accepted that occasionally occurring turbulence eddies or other flow irregularities will cause the yawing moments on the turbine rotor to exceed the capacity of the yaw drive system for short periods of time. In such situations the yaw drive system cannot carry out yawing in a desired direction and may even be forced backwards in the opposite direction. Even if it is accepted that a yaw drive system is not rated for the worst case scenarios such a system tends to be large, heavy and expensive.
When the turbine has been oriented to the wind the large yawing moments which occur on the wind turbine rotor will tend to move the rotor out of alignment with the wind. Such movement may have the character of a rocking or oscillating movement that gradually increases misalignment between the rotor shaft and the wind direction. It may also be of a more unilateral direction in which case the yaw system may quickly slide to one side.
In the past, such gradual or sudden misaligniments were prevented by using yaw drive systems with built in brakes which either passively brake the yaw system, for example by self-locking worm gears or spring tensioned friction brakes, or actively brake the gear system in the form of electromagnetic or hydraulic friction brakes or in the form of electromagnetic or hydraulic motor brakes. However, these arrangements cause two difficulties. At first, the unavoidable play in the gearing between the geared ring fixed to the tower and the pinions of the yaw drives may cause an oscillating movement and lead to a hammering of the gear mesh which is detrimental to the geared parts. Secondly, the yaw drives will have to be rated to absorb the highest yawing moment peaks.
Various brake systems have been proposed. One example can be seen in Japanese patent application JP-A-08082277 which discloses a hydraulic brake joined to the frame of the nacelle which acts on a disc joined to the tower.
Further examples are described in U.S. Pat. Nos. 4,966,525 and 5,035,575 in which two yaw drives are used. When the rotor shaft is aligned with the wind and the yaw system is desired to be stationary the two drives act in the opposite rotational direction, thereby locking the yawing system up to a certain well defined capacity that is determined by the electronic drive arrangement. When yawing is desired, the two yaw drive act in the same direction.
European patent EP 0 945 613 B1 discloses a yawing system where the machine frame in the nacelle rotates on a ball bearing relative to the tower. It includes a continuous passive braking system preventing the motor from being forced to operate due to small gusts of wind. To this end, it comprises friction plates on the frame which are pushed against a supporting ring in the tower on which the frame rotates by means of springs. These plates can be placed in the upper track or in the lower track of the ring, or in a radial direction on a cylindrical surface joined to the tower. The braking force is passive, i.e. it is adjusted during the assembly of the nacelle, and the braking is always operating either against small gusts of wind or against the desired movement of the orientation of the rotor. Electrically driven disc braking devices are also described in EP 0 945 613 B1.